Disc drives are data storage devices that store digital data in magnetic or optical form on a rotating storage medium called a disc. Modem magnetic disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Each storage surface of a disc is divided into several thousand tracks that are tightly packed concentric circles. The tracks are typically numbered starting from zero at the track located outermost the disc and increasing for tracks located closer to the center of the disc. Each track is further broken down into data sectors and servo bursts. A data sector is normally the smallest individually addressable unit of information stored in a disc drive and typically holds 512 bytes of information plus additional bytes for internal use by the drive for track identification and error correction. This organization of data allows for easy access to any part of the discs.
Generally, each storage surface of a disc in a disc drive has associated with it a head for writing and reading data to or from a sector. Each head is mounted at the distal end of an actuator arm that extends toward the disc and pivots about a bearing shaft assembly. The pivoting of the actuator arm is driven by a connected voice coil motor in the disc drive. A read element (or a reader) and a write element (or a writer) are mounted on each head. The reader and writer are separated both laterally and along the longitudinal axis of the actuator arm. The head skew angle, which is the angle between a tangential line to a track and the line drawn along the longitudinal axis of the actuator arm, changes as the head moves from the inner diameter to the outer diameter of the disc, and vice versa. The combination of the separation and the varying head skew angle causes the radial distance between the path of the reader on the disc and the path of the writer on the disc to be variable as the head moves from the inner diameter to the outer diameter of the disc, and vice versa. This varying radial distance between the reader and the writer at any given track is known in the art as the magneto-resistive offset (MRO).
In general, the data storage format of a track is comprised of an alternating sequence of control fields and data fields on a track. Control fields, such as servo fields and address marks, are permanently written to the disc during manufacture and are subsequently read by the disc drive controller to ensure proper positioning of the head, for error correction, and generally to monitor and control the operation of the drive. The data fields store user data and are routinely read and written to during drive operation. There are two common methods for positioning control and data fields on a track. The first method is to write both the control and data fields in line and as close to the center of the track as possible. The second method is to write the data fields at an offset from the control fields in order to take into account the presence of the MRO.
The basic difference between the first method and the second method is that the first method requires a micro minijog of the actuator arm during a write operation whereas the second method requires a micro minijog of the actuator arm during a data read operation. For example according to the first method, during a write operation, the reader first reads the address marks and compares them to the target address. If the address read from an address mark matches the target address, the writer writes the data in the data field. However, as soon as a target data field has been identified, the actuator arm must perform a minijog to center the writer over the data field so that the writer can write data centered in the target data field. But during a data read operation, no minijog is needed to center the reader over the data sector as the data fields and control fields are in line.
The second method is just the opposite. In the second method, when the reader is located over the control fields, the writer is also positioned over the data field (as the data field and the control fields are offset by the MRO) and no micro minijog of the actuator arm is required during a write operation. However, just the opposite is required during a read operation, the actuator arm is required to perform a micro minijog to center the reader over the data field after reading a control field.
Accurate measurement of the MRO is crucial since it will impact the disc drive track registration performance. For example, if the actual MRO at a given track is different from the MRO used by the disc drive when offsetting the head, then there is a greater likelihood of a read error due to the read head not being close enough to the data. This is referred to as track misregistration (TMR). TMR generally refers to position errors of the head between the target head position and the actual head position influenced by external disturbances such as disc flutter, runouts, disc vibrations, etc. The reader can read good data (i.e., data that contains no bit error or recoverable bit errors) only on small a portion of the track pitch (or width) of the track, and this portion of the track pitch is generally referred to as the off-track capability (OTC) of the head. For example, the OTC of a disc drive may only be about 10% of the track pitch. Thus, the reader or the writer must be positioned within the OTC (i.e., within the 10% of the track pitch) in order to successfully read information from or write data to the track.
If the MRO used by the disc drive for a specific track is inaccurate, one outcome is that the target head position may not be within the OTC of the head. The other outcome is that the target head position may not be located at the center of the OTC although it may be within the OTC of the head. In such a case, the target head position would still allow the reader to successfully read good data written on the track but would not provide optimal protection against the TMR. This is because the target head position would be located closer to one of the two edges of the OTC, and there exists higher probability that an external disturbance might displace the head beyond the OTC of the head.
Existing methods of calculating MRO are based on the assumption that, for each zone, the tracks are perfectly concentric and that the track pitch is evenly distributed within the zone. Slight variations in track pitch are assumed to be insignificant in comparison to the average track width. These assumptions become less and less valid as disc drive designs continue to incorporate increasing tracks per inch (TPI). Track pitch, inversely proportional to TPI, continues to decrease as disc drive technology advances. Variation in track pitch from track to track has become more critical in disc drive operation since fine jittering during servo writing will cause a higher percentage of disc drive certification failures for high TPI drives with incorrect MROs.
Accordingly there is a need for methods of more precisely calculating MRO that can be used effectively on disc drives with increasing TPI.